An analysis of the global array of ocean island volcanics shows that carbonatites only
form in those hotspots that have the lowest Si- and highest alkali-contents among their primitive
melts, such as the Cape Verde and Canary (Islands) hotspots. Fractionated melts from
these two hotspots reach, at any given SiO2, several wt% higher total alkali contents than
for ocean islands without carbonatites. This is because their strongly silica-undersaturated
primitive melts fractionate at low SiO2 to high alkali contents, driving the evolving melt
into the silicate-carbonatite miscibility gap. Instead, moderately alkaline magmas fractionate
toward the alkali-feldspar thermal divide and do not reach liquid immiscibility. Low
SiO2 and high alkalis are the combined result of comparatively deep and low-degree mantle
melting, the latter is corroborated by the highest high-field-strength and rare earth element
concentrations in the Cape Verde and Canary primitive melts. CO2 in the source facilitates
low melt SiO2, but enrichment in CO2 relative to other hotspots is not required. The oceanic
hotspots with carbonatites are among those with the thickest thermal lithosphere supporting
a deep origin of their asthenospheric parent melts, an argument that could be expanded to
continental hotspot settings.